Chapter 21 Somatic Senses

Sensory Modalities Different types of sensations touch, pain, temperature, vibration, hearing, vision Each type of sensory neuron can respond to only one type of stimuli Two classes of sensory modalities general senses somatic are sensations from body walls visceral are sensations from internal organs special senses smell, taste, hearing, vision, and balance

Process of Sensation Sensory receptors demonstrate selectivity respond to only one type of stimuli Events occurring within a sensation stimulation of the receptor transduction (conversion) of stimulus into a graded potential vary in amplitude and are not propagated generation of impulses when graded potential reaches threshold integration of sensory input by the CNS

Sensory Receptors Selectively respond to only one kind of stimuli Have simple or complex structures General Sensory Receptors (Somatic Receptors) no structural specializations in free nerve endings that provide us with pain, tickle, itch, temperatures some structural specializations in receptors for touch, pressure & vibration Special Sensory Receptors (Special Sense Receptors) very complex structures---vision, hearing, taste, & smell

Classification of Sensory Receptors Structural classification Type of response to a stimulus Location of receptors & origin of stimuli Type of stimuli they detect

Structural Classification of Receptors Free nerve endings bare dendrites pain, temperature, tickle, itch & light touch Encapsulated nerve endings dendrites enclosed in connective tissue capsule pressure, vibration & deep touch Separate sensory cells specialized cells that respond to stimuli vision, taste, hearing, balance

Structural Classification Compare free nerve ending, encapsulated nerve ending and sensory receptor cell

Classification by Location Exteroceptors near surface of body receive external stimuli hearing, vision, smell, taste, touch, pressure, pain, vibration & temperature Interoceptors monitors internal environment (BV or viscera) not conscious except for pain or pressure Proprioceptors muscle, tendon, joint & internal ear senses body position & movement

Classification by Stimuli Detected Mechanoreceptors detect pressure or stretch touch, pressure, vibration, hearing, proprioception, equilibrium & blood pressure Thermoreceptors detect temperature Nociceptors detect damage to tissues Photoreceptors detect light Chemoreceptors detect molecules taste, smell & changes in body fluid chemistry

Adaptation of Sensory Receptors Change in sensitivity to long-lasting stimuli decrease in responsiveness of a receptor bad smells disappear very hot water starts to feel only warm potential amplitudes decrease during a maintained, constant stimulus Receptors vary in their ability to adapt Rapidly adapting receptors (smell, pressure, touch) adapt quickly; specialized for signaling stimulus changes Slowly adapting receptors (pain, body position) continuation of nerve impulses as long as stimulus persists

Somatic Tactile Sensations Touch crude touch is ability to perceive something has touched the skin discriminative touch provides location and texture of source Pressure is sustained sensation over a large area Vibration is rapidly repetitive sensory signals Itching is chemical stimulation of free nerve endings Tickle is stimulation of free nerve endings only by someone else

Meissner’s Corpuscle Dendrites enclosed in CT in dermal papillae of hairless skin Discriminative touch & vibration-- rapidly adapting Generate impulses mainly at onset of a touch

Hair Root Plexus Free nerve endings found around follicles, detects movement of hair

Merkel’s Disc Flattened dendrites touching cells of stratum basale Used in discriminative touch (25% of receptors in hands)

Ruffini Corpuscle Found deep in dermis of skin Detect heavy touch, continuous touch, & pressure

Pacinian Corpuscle Onion-like connective tissue capsule enclosing a dendrite Found in subcutaneous tissues & certain viscera Sensations of pressure or high-frequency vibration

Thermal Sensations Free nerve endings with 1mm diameter receptive fields on the skin surface Cold receptors in the stratum basale respond to temperatures between 50-105 degrees F Warm receptors in the dermis respond to temperatures between 90-118 degrees F Both adapt rapidly at first, but continue to generate impulses at a low frequency Pain is produced below 50 and over 118 degrees F.

Pain Sensations Nociceptors = pain receptors Free nerve endings found in every tissue of body except the brain Stimulated by excessive distension, muscle spasm, & inadequate blood flow Tissue injury releases chemicals such as K+, kinins or prostaglandins that stimulate nociceptors Little adaptation occurs

Types of Pain Fast pain (acute) Slow pain (chronic) occurs rapidly after stimuli (.1 second) sharp pain like needle puncture or cut not felt in deeper tissues larger A nerve fibers Slow pain (chronic) begins more slowly & increases in intensity aching or throbbing pain of toothache in both superficial and deeper tissues smaller C nerve fibers

Localization of Pain Superficial Somatic Pain arises from skin areas Deep Somatic Pain arises from muscle, joints, tendons & fascia Visceral Pain arises from receptors in visceral organs localized damage (cutting) intestines causes no pain diffuse visceral stimulation can be severe distension of a bile duct from a gallstone distension of the ureter from a kidney stone Phantom limb sensations -- cells in cortex still

Referred Pain Visceral pain that is felt just deep to the skin overlying the stimulated organ or in a surface area far from the organ. Skin area & organ are served by the same segment of the spinal cord. Heart attack is felt in skin along left arm since both are supplied by spinal cord segment T1-T5

Pain Relief Aspirin and ibuprofen block formation of prostaglandins that stimulate nociceptors Novocaine blocks conduction of nerve impulses along pain fibers Morphine lessen the perception of pain in the brain.

Proprioceptive or Kinesthetic Sense Awareness of body position & movement walk or type without looking estimate weight of objects Proprioceptors adapt only slightly Sensory information is sent to cerebellum & cerebral cortex from muscle, tendon, joint capsules & hair cells in the vestibular apparatus

Muscle Spindles Specialized intrafusal muscle fibers enclosed in a CT capsule and innervated by gamma motor neurons Stretching of the muscle stretches the muscle spindles sending sensory information back to the CNS Spindle sensory fiber monitor changes in muscle length Brain regulates muscle tone by controlling gamma fibers

Golgi Tendon Organs Found at junction of tendon & muscle Consists of an encapsulated bundle of collagen fibers laced with sensory fibers When the tendon is overly stretched, sensory signals head for the CNS & resulting in the muscle’s relaxation

Somatic Sensory Pathways First-order neuron conduct impulses to brainstem or spinal cord either spinal or cranial nerves Second-order neurons conducts impulses from spinal cord or brainstem to thalamus--cross over to opposite side before reaching thalamus Third-order neuron conducts impulses from thalamus to primary somatosensory cortex (postcentral gyrus of parietal lobe)

Posterior Column-Medial Lemniscus Pathway of CNS Proprioception, vibration, discriminative touch, weight discrimination & stereognosis Signals travel up spinal cord in posterior column Fibers cross-over in medulla to become the medial lemniscus pathway ending in thalamus Thalamic fibers reach cortex

Spinothalamic Pathways Lateral spinothalamic tract carries pain & temperature Anterior tract carries tickle, itch, crude touch & pressure First cell body in DRG with synapses in cord 2nd cell body in gray matter of cord, sends fibers to other side of cord & up through white matter to synapse in thalamus 3rd cell body in thalamus projects to cerebral cortex

Somatosensory Map of Postcentral Gyrus Relative sizes of cortical areas proportional to number of sensory receptors proportional to the sensitivity of each part of the body Can be modified with learning learn to read Braille & will have larger area representing fingertips

Sensory Pathways to the Cerebellum Major routes for proprioceptive signals to reach the cerebellum anterior spinocerebellar tract posterior spinocerebellar tract Subconscious information used by cerebellum for adjusting posture, balance & skilled movements Signal travels up to same side inferior cerebellar peduncle

Somatic Motor Pathways Control of body movement motor portions of cerebral cortex initiate & control precise movements basal ganglia help establish muscle tone & integrate semivoluntary automatic movements cerebellum helps make movements smooth & helps maintain posture & balance Somatic motor pathways direct pathway from cerebral cortex to spinal cord & out to muscles indirect pathway includes synapses in basal ganglia, thalamus, reticular formation & cerebellum

Primary Motor Cortex Precentral gyrus initiates voluntary movement Cells are called upper motor neurons Muscles represented unequally (according to the number of motor units) More cortical area is needed if number of motor units in a muscle is high vocal cords, tongue, lips, fingers & thumb

Direct Pathway (Pyramidal Pathway) 1 million upper motor neurons in cerebral cortex 60% in precentral gyrus & 40% in postcentral gyrus Axons form internal capsule in cerebrum and pyramids in the medulla oblongata 90% of fibers decussate(cross over) in the medulla right side of brain controls left side muscles Terminate on interneurons which synapse on lower motor neurons in either: nuclei of cranial nerves or anterior horns of spinal cord Integrate excitatory & inhibitory input to become final common pathway

Details of Motor Pathways Lateral corticospinal tracts cortex, cerebral peduncles, 90% decussation of axons in medulla, tract formed in lateral column. skilled movements hands & feet Anterior corticospinal tracts the 10% of axons that do not cross controls neck & trunk muscles Corticobulbar tracts cortex to nuclei of CNs ---III, IV, V, VI, VII, IX, X, XI & XII movements of eyes, tongue, chewing, expressions & speech

Location of Direct Pathways Lateral corticospinal tract Anterior corticospinal tract Corticobulbar tract

Paralysis Flaccid paralysis = damage lower motor neurons no voluntary movement on same side as damage no reflex actions muscle limp & flaccid decreased muscle tone Spastic paralysis = damage upper motor neurons paralysis on opposite side from injury increased muscle tone exaggerated reflexes

Final Common Pathway Lower motor neurons receive signals from both direct & indirect upper motor neurons Sum total of all inhibitory & excitatory signals determines the final response of the lower motor neuron & the skeletal muscles

Basal Ganglia Helps to program automatic movement sequences walking and arm swinging or laughing at a joke Set muscle tone by inhibiting other motor circuits Damage is characterized by tremors or twitches

Basal Ganglia Connections Circuit of connections cortex to basal ganglia to thalamus to cortex planning movements Output from basal ganglia to reticular formation reduces muscle tone damage produces rigidity of Parkinson’s disease

Cerebellar Function Aspects of Function learning coordinated & skilled movements posture & equilibrium 1. Monitors intentions for movements -- input from cerebral cortex 2. Monitors actual movements with feedback from proprioceptors 3. Compares intentions with actual movements 4. Sends out corrective signals to motor cortex

Spinal Cord Injury Damaged by tumor, herniated disc, clot or trauma Complete transection is cord severed resulting loss of both sensation & movement below the injury Paralysis monoplegia is paralysis of one limb only diplegia is paralysis of both upper or both lower hemiplegia is paralysis of one side quadriplegia is paralysis of all four limbs Spinal shock is loss of reflex function (areflexia) slow heart rate, low blood pressure, bladder problem reflexes gradually return